Alvaro has put together a post that lays out some of the design patterns that the Symfony framework uses to get the job done:

Much hasbeensaid this days about how modern web frameworks interpret the MVC architectural pattern. I'm my case those articles made me rethink how I use a framework, specially Symfony. This lead me to start a study on which patterns come into play while we develop a Symfony application. So while adding new features or refactoring existing ones I will know which class is in charge off doing the job.

Some of the design patterns mentioned include the Front Controller, Contaxt Object, Helper Object, Active Record and Single Table Inheritance. He includes brief examples of each of them in use (no code but plenty of description).